How do bacteria, viruses, and protozoa enter wastewater?

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Aerial view of wastewater treatment basins with circular clarifiers; overlaid toilet graphic and arrow with text reading: “Where does it go?” — wastewater treatment All the sewers in your neighborhood end up here.

Pathogens, including bacteria, viruses, and protozoa, primarily enter wastewater through domestic sewage. Because these microorganisms are excreted in feces, sewage collected from cities and towns is inherently likely to contain various pathogens, which pose a direct threat to public health.1

While these organisms are present in the influent, it is important to note that wastewater treatment processes are specifically designed to manage them. In secondary treatment, for example, protozoans play a beneficial role by grazing on bacteria, which helps clarify the effluent. Ultimately, treatment plants employ disinfection as a final step—typically using chlorine or ultraviolet radiation—to destroy remaining pathogens before the treated water is discharged into the environment.23